1. Field of the Invention
The present invention relates to a waveform converting device for removing a carrier signal for example in an infrared remote control light receiving unit.
2. Description of the Prior Art
A conventional prior art, waveform converting device is described with reference to FIGS. 1 and 2.
In FIG. 1, there is shown a conventional waveform converting circuit for removing a carrier signal in an infrared remote control light receiving circuit from a pulse position modulation (referred to as PPM hereinafter) signal of which amplitude is modulated at the rate of 100 percent.
In the case that a data signal "1000", as shown in FIG. 2, is transmitted, an amplitude modulation (referred to as AM hereinafter) of a PPM signal representing the data "1000" is performed at the rate of 100% by a carrier signal with a frequency of 38 kHz and the modulated PPM signal is generated from an infrared remote control device (not shown). In a light receiving unit, the modulated PPM signal generated from the infrared remote control device is received as is entered as an input signal voltage V.sub.in. The input signal voltage V.sub.in is applied into first and second emitter follower circuits 11 and 12 respectively. The first emitter follower circuit 11 is composed of a common-emitter type transistor Q.sub.21 the emitter of which is connected to a first constant current unit 13 for supplying a constant current I.sub.11, so that the impedance conversion of the input signal voltage V.sub.in is performed. The second emitter follower circuit 12 comprises a common-emitter type transistor Q.sub.22 the emitter of which is connected to a second constant current unit 14 through a resistor R.sub.11 so as to supply a constant current I.sub.12, so that the impedance conversion of the input signal voltage V.sub.in is performed. Accordingly, in the second emitter follower circuit 12, the wave level of the input signal voltage V.sub.in is shifted down in correspondence with a voltage drop in the resistor R.sub.11. The voltage drop in the resistor R.sub.11 can be considered constant because of the constant current I.sub.12, if the current flowing toward the output side is ignored.
The output signal of the first emitter follower circuit 11 is transmitted to a minus input terminal of a comparator 16 through a first integrating circuit 15 which is composed of a resistor R.sub.12 and a capacitor C.sub.11. The circuit 15 integrates the input signal voltage V.sub.in. As the time constant of the first integrating circuit 15 is long enough, the first integrating circuit 15 outputs a signal at a generally constant level representing the input level. The output signal of the second emitter follower circuit 12 is transmitted to a plus input terminal of comparator 16 through a resistor R.sub.13. The comparator 16 compares the minus input signal A transmitted from the first emitter follower circuit 11 with the plus input signal B transmitted from the second emitter follower circuit 12 as shown in FIG. 2, and when the level of the input signal B which is the level shifted input voltage V.sub.in exceeds the level of the input signal A, the comparator 16 outputs a high level of two-value signal. The output signal of the comparator 16 is transmitted through a second integrating circuit 17 so as to generate an output signal voltage V.sub.out of the waveform converting device. The second integrating circuit 17 comprises a capacitor C.sub.12 and integrates the output signal of the comparator 16. Accordingly, as shown in FIG. 2, the output signal voltage V.sub.out of the waveform converting device is recovered to the original PPM signal and the carrier signal can be removed.
In such a conventional waveform converting device, as described above as the time constant of the first integrating circuit 15 must be made long, it is necessary to provide a capacitor C.sub.11 of large capacitance, for example, with a value of 10 .mu. F. In order to obtain such a large capacitance, it is necessary to use an electrolytic capacitor. For this reason, there have been problems that, in case of fabricating the waveform converting circuit in an integrated circuit form, it is difficult to miniaturize a light receiving unit because the electrolytic capacitor must be externally attached to the device, and that the cost of the light receiving unit can not be reduced because of the high cost of the electrolytic capacitor and the problems of the connecting the capacitor.
In addition, even though the capacitance of the capacitor C.sub.11 is made small, if the resistance of the resistor R.sub.12 is made large in the first integrating circuit 15, the same time constant can be obtained. However, if the resistance of the resistor R.sub.11 is made large and the capacitance of the capacitor C.sub.11 is made small, the impedance of the first integrating circuit 15 is increased. Therefore, if the constant current I.sub.11 obtained by the first constant current unit 13 is reduced, also the constant current I.sub.12 obtained by the second constant current unit 14 must be reduced according to the reduction of the constant current I.sub.11. Therefore, the resistance of the resistor R.sub.11 must be made large in order to obtain a predetermined level of voltage shifted down in the second emitter follower circuit 12. Therefore, there has been also a problem that, in the case of fabricating the waveform converting circuit in the integrated circuit form, since a large area on a chip is occupied by the resistor R.sub.11 in order to obtain a large resistance of the resistor R.sub.11, the integration of the waveform converting circuit is made difficult.